There’s a mysterious new planet hiding in our solar system. What do we know about it so far?

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Scientists have narrowed down the location of the mysterious Planet Nine.

Wait, isn’t Pluto the ninth planet?

Not since 2006, when the International Astronomical Union (IAU) downgraded it to a dwarf planet. Ever since, astronomers have been lobbying the IAU to change its mind.

The latest attempt was to say that because Pluto passes the Star Trek test—that people can say what’s a planet when they see one—it should be reinstated to be the ninth planet. The IAU hasn’t budged, so we are currently at eight planets in the solar system and many dwarf planets.

But there may be another planet that could be the ninth planet?

Yes. Researchers at the California Institute of Technology (CalTech) announced in January that there may indeed be a mysterious planet which could explain some of the mysteries of the solar system that have yet gone unsolved.

A mystery could solve many other mysteries? I’m confused.

Let’s break it down. The main reason Pluto was downgraded to the status of a dwarf planet was because scientists had found many celestial bodies similar to Pluto, which were orbiting our sun. However, their orbits were skewed to one side of the sun and scientists couldn’t explain why.

(Caltech/R. Hurt (IPAC))

Applying simple planetary physics, they figured, that there must be a planet, probably more massive than our own Earth, which would counter-balance the orbits of all these smaller celestial bodies. Current calculations show that the planet should be about 10 times the mass of the Earth, and its huge orbit should take it about 20,000 years to complete.

It’s just a hypothesis then. Why trouble us?

It sure is, but the evidence is mounting that Planet Nine might actually be real. That’s just what CalTech researchers were hoping would happen after their announcement. Astronomers around the world are pointing their telescopes and radio antennas to try to look for Planet Nine.

In February, researchers at Nice Observatory in France did something clever. They used data from the Cassini probe, which has been exploring Saturn and its moons for the last 10 years, to understand how all the large bodies of the solar system behave. They added the mysterious Planet Nine to the mix, and the computer simulations of the solar system eliminated half the orbits that CalTech researchers had suggested.

Now researchers at the Harvard-Smithsonian Center for Astrophysics have gone a step further. They used the same data from Cassini but ran a different set of calculations (looking for many more kinds of planets) and they have further narrowed down where Planet Nine might be hiding.

Planet-hunters around the world are giddy with excitement. “I’m just dropping everything to work my hardest to do this search,” David Gerdes, a cosmologist at the University of Michigan, told the New Scientist.

If it’s real, why has it taken us so long to find it?

Many reasons. First, before we found all these other small celestial bodies, the solar system’s eight planets were enough to explain everything we had observed. The new discoveries changed our understanding and created the need for another explanation. Second, its huge orbit means that it’s probably not come anywhere close to our sun during the period when humans built the technology to scour the skies. Third, it may be too cold to give out a heat signature and too dark to reflect light, both of which could have warned astronomers of something.

OK. So how are we going to find it?

We’ll need many eyes in the skies, many people on computers looking at the data we’ve already collected, and, finally, we’ll need to put together a whole slew of solid evidence that all the data irrefutably shows that Planet Nine exists.

If we do find it, will Planet Nine have anything special in store for us?

“YES!” any astronomer will shout. We’ve not found a new planet since the 1930 discovery of Pluto (which then we downgraded in 2006). There is also the tantalizing possibility that our sun stole Planet Nine from a passing star a long time ago.

So that’s it for Pluto then. Nobody is going to reinstate it to being a planet again, right?